Crystallite separation

Fig. 17. Structural diagram (51) for sputtered layers. Zone 1 is a porous stmcture consisting of tapered crystallites separated by voids, Zone 2 shows...

Nylon fibers are semicrystalline, that is, they consist of crystallites separated by amorphous regions. Hydrogen bonding is an important secondary valence interaction in nylon-6 and nylon-6,6. Individual chains in the microcrystalline regions of nylons are held together by hydrogen bonds. Nylons are resistant to aqueous alkali but deteriorate more readily on exposure to mineral acids. 

Porous structure consisting of tapered crystallites separated by voids... 

Since these hexacyano complexes are hydrated salts, aqueous solutions are used. To prepare the saturated Sm[Fe(CN)6] solution 10 mL of 1 M neutral SmCl3 solution is first added, followed by the same amount of 1 M K3[Fe(CN)6] solution, to 500 mL of distilled water kept at 65°. In this way a supersaturated solution is obtained that (if everything is clean) can remain in this metastable state for hours before the first crystallites separate. To accelerate the procedure, microcrystals of Sm[Fe(CN)6] 4H20 are prepared by mixing 1 M solutions of 1 mL of SmCl3 and 1 mL of K3[Fe(CN)6], The tiny seed crystals that are obtained are used to initiate precipitation in the growth solution. After 10 minutes, still at 65°, the solution is filtered and 50 mL of hot dis-... 

Figure 3a. Platinum on Carbon catalysts in 190 °C phosphoric add. Compilation of specific activities for oxygen reduction versus platinum crystallite separation from Watanabe7 et al. M Giordano et al. —maximum values only24 ] Buchanan et aL 8[0] and Giordano et al. 15 16[D]. Function of upper tine is abestfittothe Buchanan values and the lower tine is a best fit to the Giordano values, (a) Data for crystallite separations up to 325 mn. (b) Data below 60 nm crystallite separations.

Figure 5 shows the influence for taking the logarithm ofthe specific activity versus the logarithm of 1/X, so that the slope defines the value of the exponent, m, which is of course, -3. All ofthe data fall in a linear band, limited at the upper level by the results of Buchanan et al.,s and at the lower by the results of Giordano et al.15,16 The results ofWatanabe-Stonehart fit between the boundaries with a directive that indicates a transition from spherical to semi-infinite diffusion due to closeness of approach of the crystallites. The size of an individual platinum crystallite also is indicated in Fig. 5. Extrapolation of the high points to zero crystallite separation gives 20 pA cm 2 Pt, which is close to that value obtained by Bett et al.1 in sulphuric acid.

Figure 5. Power function of log specific activities versus the reciprocal of the platinum crystallite separation on the carbon surface. Watanabe et at [A] Buchanan et aZ.[0] and Giordano etal.15 16 [ ]. The broad arrow on the abscissa denotes the average diameter of a single platinum crystallite.

Monocrystalline Quasicrystalline Polycrystalline Semicrystalline Amorphous and glassy state LRO NoncrystaUographic rotational symmetry, no LRO Crystallites separated by grain boundaries Crystalline regions separated by amorphous regions No LRO, no rotational S3fmmetry, does possess short-range order (SRO)... 

In view of the relative thermal stabilities of the oxides (Rh203 > PdO Pt02), it is not surprising that under oxidizing conditions and at temperatures higher than 870 K, metallic Pt particles exist in combination with Rh oxide 132,133) and that PdO crystallites separate from Pt crystallites in Pd-Pt alloys in the temperature range 670-870 K 134). A more complicated behavior was found for Pd-Rh 135). 

The objective in the preparation of supported catalysts is to have the catalytically active crystallites separated. When this is the case the only way sintering can occur is if the catalyst particulates migrate across the support surface or if there is a vapor phase sintering promoter present in the reaction medium. The lower the catalyst load and the higher the support surface area, the less likely that sintering will take place. The migration ability of the catalytically active species depends primarily on the strength with which it is bonded to the support. If there is a weak interaction the catalyst particles can move across the support... 

According to a different theoretical approach, the kinetics of oxygen on Pt catalysts were interpreted on the basis of the separation distance between metal nanoparticles. Pt crystallites, separated by less than about 20 nm, were observed to exhibit lower electrocatalytic activity because of either diffusion or a mutual... 

Microscopy is the most appropriate technique for studying the kinetics of nucleation. The shapes, sizes, textures and distributions of nuclei can be determined and the kinetics of nucleation can be distinguished fi om the kinetics of growth. Details of the intranuclear material, which is often porous with small crystallites separated by fine channels that provide routes for escape of product gas, may be discemable. Changes in particle-size, topochemical relationships and the possibility of melting of the solid reactant can also be recognized. 

Figure Ila shows how an ideal cosine amplitude modulation of the RF carrier wave could be approximated by a rectangular RF pulse scheme, which is much easier to implement. Such a scheme comprises of pulses with alternating phases of 0° and 180° and is referred to as FAM. As was already mentioned, the modulation frequency should be tuned such that Vm matches tq, at least during part of the excitation. Due to the sample spinning, the quadrupolar splitting of many crystallites will pass through the v n value. It was shown that mismatches between and the powder i/qS do not create large phase distortions and simultaneous adiabatic and direct coherence transfer processes result in relatively pure MQ SQ transfers. By pure we mean that no significant phase dispersions are observed when looking at the transfer of each crystallite separately.

Synthesis of ammonia. The synthesis reaction is dependent on the conditions of equilibrium and the kinetics of the reaction. The latter is dictated by the efficacy of the catalyst, which in turn is chosen because of its cheapness and activity. Iron is the only realistic catalyst, but its activity can be greatly increased by the use of suitable promoters. It is prepared by melting iron oxide, refractory oxides such as potassium and aluminium oxides. A solid sheet forms on cooling, and is broken down into 5-10 mm lumps. The whole is then reduced in the ammonia synthesizer, where the oxide is converted to iron crystallites separated by the refractory oxides and covered in part by KOH as a promoter. The KOH can enhance the reactivity twofold. This catalyst must be used within the temperature range 400°-540 °C. Below this the catalyst becomes uneconomically inactive above, it sinters and loses surface area. An improved iron catalyst of higher activity and longer life is a feature of the AMV process. It is important to note that much of the reason for improved and continued activity is due to the careful removal of poisons such as CO, CO2, and H2S. 

The number of bonds ( ) in a typical Gaussian amorphous chain sequence, which links the two adjacent crystallites separated by distance (Fig. 13.3), is given by ... 

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