Crystal growth and phase transformations

Year Vapor Phase Processes Fiber Chapter Ref. 

1961 Metal particle catalyzed CVD Semiconductor, ceramic, carbon 2.2 [1] 

1983 Self catalyzed rice hull processes SiC and SiN microwhiskers 2.2 [14] 

1983 Metal cat. caibothermal reduction Ultrahigh strength SiC whiskers 2.2 [29] 

1991 Self-cat. caibothermal reduction SiC whiskers, in-situ composites 2.2 [30] 

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One of the most commonly applied SR-based scattering methods is powder X-ray diffraction, usually accompanied by Rietveld profile fitting of the diffraction pattern (e.g., Hazemann et al. 1991 Parise 1999 Lee et al. 2001). Special in situ reaction cells have been designed to study crystal growth and phase transformations in aqueous solutions at ambient to moderate temperatures ( 200°C) using X-ray diffraction (Cahill et al. 1998). Such cells have been used to study pyrite growth from aqueous solutions (Cahill et al. 

At low and medium supersaturations, hydrophilic cations form different crystal hydrates by heterogeneous nucleation and subsequent crystal growth and phase transformation. Dehydration curves give information on the modes of water incorporation resulting from different modes of crystallization. A useful application of thermal analysis is the analytical approach by determining the mass loss due to dehydration, it was possible to quantitatively determine the proportion of different calcium oxalate hydrates in mixtures, which have been qualitatively analyzed by other techniques (X-ray powder diffraction, IR spectroscopy, etc.). The method yielded excellent results in studies of the kinetics of phase transformation and has been successfully used to demonstrate the potential of surfactant micelles to control the nature of the crystallizing phase. 

Gallardo Amores, J.M., Sanchez Escribano, V., and Busca, G. Anatase crystal growth and phase transformation to rntUe in high-area Ti02, Mo03-Ti02 and related catalytic materials. J. Mater. Chem. 1995, 5, 1245-1249. 

A titania (TiOa) sample with a large surface area (300 m g ) was chemically modified with 3-aminopropyltrimethoxysilane (APS) under a reflux of toluene. The thermal stability of the modified TiOa (APS-Ti02) and the adsorptivity of metavanadate anion (VO3 ) on APS-TiOz from an aqueous solution were investigated. Modification with APS suppressed crystal growth and transformation of anatase crystallite to rutile upon calcination, and the anatase phase was preserved even after calcination at 1000°C, while transformation to rutile in the unmodified TiOz samples was observed at around 800°C. Since there was little crystal growth in APS-TiOz, it possessed a large surface area of 205 m g after calcination at 700°C. The amount of adsorbed on APS-TiOz was ca. 

Theoretical and experimental studies of the role of solvent on polymorphic crystallization and phase transformations abound in the literature of the last few years and some pertinent examples are described here. For solvent-mediated transformations, the driving force is the difference in solubility between different polymorphs. An important earlier paper on the kinetics of such phase transformations [51 ] described a model featuring two kinetic processes in sohd to solid phase changes via a solution phase, namely dissolution of the metastable phase and growth of the stable one. 

We shall study the following topics twinning elements, mechanical twin formation, twin junction and twin intersection, twin by crystal growth and grain boundaries, the real structure of B, and A, B phase transformation and the domains of B by phase transformation. 

Zeolite crystallization is a phase transformation process, since an amorphous alumino-silicate gel phase usually forms quickly after mixing the reagents in the appropriate concentrations. Clear solution syntheses have been reported, but the yield from them is typically not sufficient to generate commercial interest. It is generally agreed that the transformation from amorphous gel to crystalline zeolite occurs through the solution phase via dissolution of the amorphous gel and crystallization of the desired zeolite phase. Consequently, the normal processes of nucleation and crystal growth must occur from within the solution. 

Fig. 2 Crystallization pathways under thermodynamic and kinetic control Whether a system follows a one-step route to the final mineral phase (pathway A) or proceeds by sequential precipitation (pathway B), depends on the free energy of activation associated with nucleation (N), growth (g), and phase transformation (T). Amorphous phases are common under kinetic conditions. Reproduced from [40] with permission of Wiley...

B. Nucleation, crystal growth, and solution-mediated phase transformation of crystal hydrates... 

B. Nucleation, Crystal Growth, and Solution-Mediated Phase Transformation of Crystal Hydrates... 

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