Nascent crystallization

All of these parameters are not under control. To deliver a more uniform distribution of crystallites, specific nucleating agents are added whilst processing. It has also been shown that many of the desirable properties are the result of small regular spherulites (or nascent crystals). 

Fifty grams of fine KgCrgO powder is dissolved in a mixture of 65 ml. of cone. HCl and 50 ml. of water (by heating to 70°C). The solution is filtered through a jacketed funnel heated with hot water. After 1-2 days, the nascent crystals are filtered off with suction, recrystallized from glacial acetic acid, and dried in a vacuum desiccator over H3SO4. 

A saturated solution of 15.67 g. of KsCrg07 and 43.43 g. of CrOa (mole ratio KgtCrOa = 1 10.15) in 40.9 ml. of water is prepared at 60°C and concentrated at this temperature to about 10 ml. The nascent crystals are separated and dried as described in the case of the trichromate. Yield about 13 g. 

The foregoing studies point to conclusions similar to those of the extensive research on induced crystallization of inorganic and organic crystals under close-packed amphiphilic Langmuir monolayers [88-92]. All these studies show that it is possible to design interfaces that will not only catalyze nucleation but will also orient the nascent crystals in a well-defined and reproducible manner. The... 

Many biological mineralization processes involve oriented nucleation strictly controlled by organized organic interfaces that interact with the nascent crystals by virtue of structural and stereochemical recognition processes [57,91,92,117],... 

We have shown that out of fifteen forms of three-dimensional crystals from ribosomal particles, grown so far in our laboratory, some appear suitable for crystallographic data collection when using synchrotron radiation at temperatures between 19 °C and —180 °C 50S subunits from H. marismortui., and from B. stearothermophilus, including the -BLl 1 mutant, and the new crystal forms from B. stearothermophilus SOS and Thermus thermophilus 30S subunits which have only recently been grown in non-volatile precipitants We also plan to continue research on biochemically modified particles, such as SOS with one tRNA and its nascent polypeptide chain (which have already been crystallized). 

By inspecting Fig. 1.10, it is obvious that the most natural cleavage plane of a Si crystal is the (111) plane or its equivalent, namely, (iTl), (111), etc. Right after cleaving, on each of the surface Si atoms, there is a broken bond, or a dangling bond, that is perpendicular to the (111) surface. Each of the dangling bond orbitals is half filled, that is, has only one electron. The nascent Si(lll) surface is thus metallic and exhibits a threefold symmetry, as shown in Fig. 1.11 (a). However, because of the large number of unsaturated bonds, such a surface is unstable. It reconstructs even at room temperature, and loses its threefold symmetry. 

Plate 5. The nascent Si(lll) surface. The Si(l 11) plane is the natural cleavage plane of the Si crystal. After cleaving, the surface immediately reconstructs into a 2X1 structure. The threefold symmetry is locally degenerated to a twofold symmetry. There are three equivalent orientations of the Si(lll)-2X1 structure. On a large scale, different orientations coexist to make a mosaic. See Feenstra and Lutz (1991) for details. Original image courtesy of R. M. Feenstra. 

Same form and parameters for crystals of a complex of 50S subunits, one tRNA molecule, and a segment (18 to 20-mers) of a nascent polypeptide chain. 

An additional elegant route to obtain disentangled UHMW-PE, and thus to control the interphase, is by direct polymerization in the reactor. In order to make UHMW-PE, a relatively low polymerization temperature is needed and a situation is easily encountered where the polymerization temperature is lower than the crystallization temperature of UHMW-PE in the surrounding medium in which the catalyst is suspended. In this situation, the growing chains on the catalyst surface tend to crystallize during the polymerization process. These UHMW-PE reactor powders, often referred to as nascent or virgin UHMW-PE, can be remarkably ductile. It was shown by Smith et al. [26] that reactor powders, in the same manner as solution-cast UHMW-PE, could be drawn easily into high-modulus structures. 

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