Amorphous crystallization

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

I feel like a white granular mass of amorphous crystals—my formula appears to be isomeric with Spasmotoxin. My aurochloride precipitates into beautiful prismatic needles. My Platinochloride develops octahedron crystals,—with a fine blue florescence. My physiological action is not indifferent. One millionth of a grain injected under the skin of a frog produces instantaneous death accompanied by an orange blossom odor. 

As a rule, the fresh precipitates are X-ray amorphous. Crystallization is accelerated at increased temperature of the mother solution crystallization also takes place during precipitate ageing. 

CAS 124-65-2. (CH3)2As00Na 3H20. Properties White, amorphous crystals or powder. Deliquescent. Melts about 60C, loses water at 120C. Soluble in water and alcohol. 

Fig. 2.17. Schematic illustration of the various quantities used to describe the growth of an amorphous interlayer. The composition profiles, chemical potential profile, etc., are shown. The quantity xA is the concentration of element A in the diffusion couple (in the text xA = 1 — x), while X, and X2 are the positions of the amorphous/crystal interfaces...

Channeling measurements have been used to study the epitaxial regrowth of Ge and Si crystals amorphized by ion implantation for a variety of crystal orientations (Csepregi et al. 1977). These studies have shown that, with the exception of (111) orientated Si crystals and samples cut within 16° of the (111) direction, the amorphous/crystal interface moves with a constant velocity toward the surface (at a fixed annealing temperature) and maintains a laterally uniform front. 

The phosphates of the incorporated metals are usually Insoluble in water. The crystallization of these phosphates or of an aluminophosphate with higher density may take place in different ways. It can be amorphous, crystallize in different structures on or beside the ALPOij-5 or SAPO-5 structure and may cause a recrystallization cristobalite- or tridymite-. like aluminophosphate. 

When the polymer has the potential to reach the three-dimensional order (12), that is, the crystalline state, the fracture behavior becomes more complex. It is well known that the distance between the macromolecular chain segments in the crystal are shorter than those located in the amorphous phase. Hence, when the material is required to dissipate any kind of energy, this would be preferably dissipated through the amorphous phase and, even more, through the zone where the chain segments are significantly constrained, as it is an amorphous/crystal interphase. 

So, if the interfacial regions have to be considered, we must differentiate the amorphous/crystal polymer interface from the amorphous polymer/mineral interface (29). By DMA measurements, a decrease in Tg matrix from 7°C for the PP/talc composites and also for the neat PP processed under similar conditions while a decrease upto 13°C was found for PP/mica composites. A higher fraction of free amorphous phase on the PP/mica system than on the PP/talc composites was evidenced. This free amorphous phase appeared to participate in the cooperative segmental free-rotation motion, well accepted (30) to be responsible for glass transition for the polymer matrix as fully discussed in Reference 29. 

Conversions of a metastable phase into a more stable phase may include the transformation of one polymorphic phase into another, the solvation of an anhydrous phase, the desolvation of a solvate phase, the transformation of an amorphous phase into a crystalline anhydrate or solvate phase, the degradation of a crystalline anhydrate or solvate phase to an amorphous phase, or in the case of digoxin, the conversion of imperfect (less crystalline, more amorphous) crystals with a high density of defects into more perfect (more crystalline) crystals with a lower density of defects. While it is straightforward to determine the equilibrium solubility of a phase that is stable with respect to conversion, the measurement of solubilities of metastable phases that are susceptible to conversion is not a trivial matter. 

Atomizers nozzle (air, pressure position, number) rotary, etc. Shape/size, size distribution of drops Trajectory of drops Flow rate/pressure drying air Temperature air inlet/outlet Flow rate/temperature product Powder yield (TS), sticking Water content, aw composition, retention, degradation,% surface structure (amorphous, crystallized) Size, density, wettability, flowabiUty, surface state Temperature, Tg, MP... 

Shah, B. Kakumanu, V.K. Bansal, A.K. Analytical techniques for quantification of amorphous/crystal-line phases in pharmaceutical solids. J. Pharm. Sci. 2006, 95(8), 1641-1665. 

Characterization of solution-mediated transformations in the amorphous state can give an insight into amorphous crystallization (Zhang et al. 2(X)9). The importance of the phase transition kinetics, molecular interpretations, and process implications has been emphasized in numerous studies (Cardew and Davey 1985 Davey et al. 1986, 1997a, 1997b Rodriguez-Homedo et al. 1992 Blagden et al. 1998). 

Figure 1. The estimate of excess Gibbs free energy (AG) against temperature suggesting a first-order transition. The solid lines (indicated as amorphous 1 and 2) represent two extreme estimates of AG for the amorphous phase and the dashed lines are extrapolation of these AG into the liquid phase. The dotted line shows AG for the liquid phase and the long-dashed is the reference crystal phase value. Tai, Tic, and Tac (dot-dashed line) represent liquid-amorphous, crystal-liquid, and oystal-amorphous phase transition temperatures, respectively. [Adapted from Donovan et al. [40] with permission.)...

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