Crystal morphologies

The shape of the crystals that are formed by a particular compound is determined by the symmetry of the unit cell of the compound and by conditions such as ion concentrations, temperature, pressure, and the space available for the formation of the crystals. The shapes of crystals are described by their forms, such as cube, tetrahedron, etc., and by the symmetry of the crystals. In the sections that follow, we will first develop a system of designating the relationship of the faces of a crystal to the crystallographic axes and then use this system to develop the general idea of a crystal form. 

One of the many unique features of crystals is crystal habits. Crystals can have different shapes and faces that make them unique and distinct from hquid or gaseous materials. In view of the focus of this book, crystal morphology means the approximate shape of crystals. Our purpose is to describe quahtatively the aspect ratios of crystals in three-dimensional space with relevance to the crystallization process. Therefore, we refer the reader to (Mullin 2001, Chapter 1) for a more thorough and comprehensive classifications of crystal morphology. 

Crystal moi-phology refers only to the appearance of crystals. It does not reflect the internal form of crystals, as described in Section 2.4. Crystals of different morphology can have the same or different forms. 

Roughly speaking, in the variety of phanuaceutical compounds, it is common to observe three types of crystals, as shown iir Fig. 2-27. 

In this book, the first type crystal is called needle-like because it has only one key dimensional length. The second type of crystal is called plate-like because it has two key dimensional lengths. The thir d type of crystal is called rod-like or cube-like because it has three key dimensional lengths. Clearly, this is only a qualitative description of the habit of crystals. 

The crystal habit gives engineers quick, useful information on the performance of the crystallization process. We will address how to utilize this information in the following sections. 

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Fig. 27. Recognition at crystal interfaces and its role in the engineering of crystal morphology and configurational assignment of molecules (176,177).

Cadmium Iodide. Two crystal morphologies exist for Cdl2 [7790-80-9] the white a-form (see Table 1) and the brown p-form. The latter... 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

Characterization. Ceramic bodies are characterized by density, mass, and physical dimensions. Other common techniques employed in characterizing include x-ray diffraction (XRD) and electron or petrographic microscopy to determine crystal species, stmcture, and size (100). Microscopy (qv) can be used to determine chemical constitution, crystal morphology, and pore size and morphology as well. Mercury porosknetry and gas adsorption are used to characterize pore size, pore size distribution, and surface area (100). A variety of techniques can be employed to characterize bulk chemical composition and the physical characteristics of a powder (100,101). 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

Docherty, R., Roberts, K.J. and Dowty, E., 1988. MORANG - a computer programme designed to aid in the deteiTnination of crystal morphology. Computer Physics Communications, 51, 423-430. 

Giorgio, S. and Kern, R., 1983. Filtrability, crystal morphology and texture paraffins and de-waxing aids. Journal of Crystal Growth, 62, 360-374. 

In the analysis of crystal growth, one is mainly interested in macroscopic features like crystal morphology and growth rate. Therefore, the time scale in question is rather slower than the time scale of phonon frequencies, and the deviation of atomic positions from the average crystalline lattice position can be neglected. A lattice model gives a sufiicient description for the crystal shapes and growth [3,34,35]. 

A procedure for proplnts is presented by J.W. French (Ref 27), who used both OM and EM (electron microscope) to study plastisol NC curing. He found that the cure time of plastisol NC is a logarithmic function of temp, and direct functions of chemical compn and total available surface area, as well as of particle size distribution. It should be noted that extensive use of statistics is required as a time-saving means of interpreting particle size distribution data. The current state-of-the-art utilizes computer techniques to perform this function, and in addition, to obtain crystal morphology data (Ref 62)... 

Kurimura, S. and Nakamura, R. (2005) Control of crystal morphology in dewetted films of thienyl dyes. Appl. Phys. A, 80, 903-906. 

Figure 10. SEM photographs of polished, etched thin sections of fossil Acropora palmata coral (after Edwards 1988). The scale bar in a is 10 microns, a depicts sample AFS-12, a last interglacial coral from Barbados. The crystal morphology in this well-preserved sample is indistingnishable from that of a modem sample (see Fig. 9b). The scale bar in b is 100 microns, b depicts sample PB-5B, a fossil coral collected from North Point Shelf on Barbados. The crystal morphology of this sample shows clear evidence of alteration, inclnding a large calcite crystal filling in a macroscopic pore (dark area in npper right portion of photograph).

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