Importance of crystallization

C M. Packer. R.H. Johnson, O.D Sherby, "Evidence for the importance of crystal- lo-graphic slip during superplastic deformation of eutectic Zinc - Aluminium". Trans.Met.Soc. AIME242, 2485, 1968... 

The central problem in organic solid-state photochemistry is the preorganization of molecules satisfying the topochemical postulates. Schmidt coined the term crystal engineering for this problem of supramolecular assembly. Indeed, the importance of crystal engineering is fundamental to areas as diverse as nonlinear optics, high- /) superconductors, and the generation of polymorph forms in pharmaceuticals. 

This chapter will not provide reviews of density and structure prediction, but rather focus on our work and its emphasis on predictions for energetic materials. Because of the importance of crystal density in the performance of energetic materials, our initial efforts were directed to this area in general and to volume additivity techniques in particular. 

Crystallization and adsorption are both widely used to perform the separation distillation is not used (except for orthoxylene separation) because of too small differences between the boiling points (Table 10.1). Despite the still high importance of crystallization, adsorption becomes the most widely used technique because of its high efficiency. The adsorbents which are used for selective adsorption of paraxylene are X or Y zeolites exchanged with adequate cations. Liquid phase Simulated Counter Current adsorption, which is the most efficient process, is generally used (1). In addition to the complexity of this process, the choice of an adsorbent selective for paraxylene is the critical point. 

There are several reports on the effects of different substituents with a given parent and the importance of crystal modification. In a study by Hashimoto (1986), several bisazo pigments based on diaminofluorenone were synthesized and characterized. Table 1 shows the results for AZO-FO and related pigments used by Ricoh in Fig. 6. The transport layer contained the hydrazone CTM-2. 

Fig. 1 Flow chart of the cause and effect relationship outlining the importance of crystal habit in dosage form design and performance.

The term crystallization describes the production of a solid, single-component, crystalline phase from a multicomponent fluid phase. The importance of crystallization lies primarily in the purification achieved during the process and in the physical properties of the product. A crystalline powder is easily handled, stable, and often possesses good flow properties and an attractive appearance. 

The great importance of crystallization lies in the three chief things which it accomplishes ... 

Craig et al. (97), Marzouk et al (98) and Sankar et al (99), revealed the importance of crystal field interactions in these compounds. As an illustration, we will describe the magnetic thermal properties of PrNis, which exhibits particularly interesting crystal field features (97). 

The following sections review a selection of characteristic recent applications of ab initio methods to the spectroscopy of ionic impurities. Our main goal is to illustrate the strengths and limitations of existing approaches to treat electron correlation and spin-orbit effects, and we will also emphasise the importance of crystal effects. 

Recently, a group of authors from the Institut Franfais du Petrole disclosed that Co catalysts can be used to produce a mixture of various alcohols (Ci-Cg) when certain rules are followed during the preparation of the catalysts. The authors stressed the importance of crystal chemistry in the strong interaction between the support and the precursor of the metal phase. Only precursor materials of certain phase composition and crystallographic structure lead to catalysts active in higher alcohol synthesis. 

CuOi t is special because it is oxygen deficient but behaves differently from Zni+ (0 it shows the importance of crystal structure. 

Garside, J., Mullin, J.W. and Das, S.N. (1973) Importance of crystal shape in crystal growth rate determinations. Industrial and Engineering Chemistry Process Design and Development, 12, 369-371. 

Frank RM, Klewansky P, Hemmerle J, Tenenbaum H. Ultrastructural demonstration of the importance of crystal size of bioceramic powders implanted into human periodontal lesions. J Clin Periodontal. 1991 Oct 18(9) 669-80. 

Crystallization is the most commonly cited mechanism for junction-zone formation in polymer gel networks. A distinction must be made between crystallization from solution resulting directly in network formation and crystallization which occurs subsequently to gelation. This distinction is not always apparent in the reports of crystallization and gelation. For example, crystallization may occur in an already phase-separated network or it may involve polymer molecules not included in the network structure. Another possibility is that a specific interaction or phase separation results in the formation of a weak network but that this network is consolidated by subsequent crystallization. Here the relative importance of the various mechanisms becomes unclear. These form the grounds for the current debate on the relative importance of crystallization and phase separation in gelation. In the examples which follow crystallization has been favoured as the primary mechanism. 

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