Crystal growth processes involved

Crystal growth process involves two fundamental steps the deposition of the first stem on the growth front ( secondary nucleation process ) and the attachment of subsequent stems in the chain on the crystal surface ( surface spreading process ). 

We have seen that the deposition of crystals from the vapor is much too slow to model by MD techniques. Most laboratory equipment for producing thin films involves relatively slow crystal growth processes, and is not suitable for direct simulation. Information on the stability and properties of thin films can be obtained by similar modeling techniques, however. We describe below some of our results that provide necessary data to find the equilibrium configuration of thin films at low temperatures. 

The exponents i and s in equations 15.13 and 15.14, referred to as the order of integration and overall crystal growth process, should not be confused with their more conventional use in chemical kinetics where they always refer to the power to which a concentration should be raised to give a factor proportional to the rate of an elementary reaction. As Mullin(3) points out, in crystallisation work, the exponent has no fundamental significance and cannot give any indication of the elemental species involved in the growth process. If i = 1 and s = 1, c, may be eliminated from equation 15.13 to give ... 

Several high-temperature methods leading to fluoridated apatites can be found in the literature they involve solid-gas reaction, pyrolysis or crystal growth processes. 

The mechanism of formation of zeolites is very complex, stemming from the diversity of chemical reactions, including various polymerization and depolymerization equilibria, nucleation and crystal growth processes. The physical and chemical nature of the reactants, which typically involve a source of aluminum and silicon along with hydroxides and salts determine the formation of zeolites. Physical effects such as aging, stirring, and temperature also play an important role. These effects lead to the complexity of zeolite formation, but are also responsible for the large number of frameworks that can be synthesized and the rich chemistry associated with this area. Cl. 21... 

The clusters transform into the crystal nuclei having ability for the growth under certain conditions. Only clusters with some critical size r = 2 Oct/Ap can become potential centers of crystallization (here Cl is the specific volume of an atom or molecule involved into the cluster, a is the specific surface energy of interphase boundary and Ap is the difference in chemical potentials for the phases) [10]. There are two main possible ways to transform the clusters into the critical crystal nuclei as a result of fluctuations [1] (1) attachment of individual atoms (molecules) to the cluster and (2) coalescence of clusters with each other. It should be noted that in principle the critical nuclei formation as a result of coalescence of individual atoms (molecules) with each other is possible too. In all these cases the clusters must grow to the size r > r, in order the start a crystal growth process. 

The electrode reaction may involve the formation of a new phase (e.g. the electrodeposition of metals in plating, refining and winning or bubble formation when the product is a gas) or the transformation of one solid phase to another (e.g. reaction (1.5)). The formation of a new phase is itself a multistep process requiring both nucleation and subsequent growth, and crystal growth may involve both surface diffusion and lattice growth. 

Crystalline growth . (1) The expansion and development of a crystal. The process involves diffusion of the crystallizing material to special sites on the surface of the crystal, incorporation of the molecules into the surface at these sites, and diffusion of heat away from the surface of the crystal. (2) The transformation of disoriented molecules, usually of the same substance, to a higher state of order. This process generally occurs rapidly for small molecules however, the process is slow for polymer molecules and is arrested at temperatures below the glass transition temperature. Hibbard MJ (2001) Mineralogy. McGraw-Hill Companies Inc., New York. 

As the synthesis proceeds at elevated temperatures, zeolite crystals are formed by a nucleation step, followed by a crystal growth step involving assimilation of alumino-silicate from the solution. The amorphous gel phase continues to dissolve, thereby replenishing the solution with alumino-silicate species. This process results in the transformation of amorphous gel to crystalline zeolite. 

The formation of a-Fe crystallite during reduction is a crystal growth process and it involves the theory and rule of crystallography. The basic requirement of this process is the formation of small a-Fe crystals without growth or conglutination of a-Fe crystals into big particles. This requirement is related with reaction rate, especially, temperature and the concentration of produced H2O. Iterative redox of a-Fe by H2O should be avoided. Thus, the low-temperature, high-space velocity and low-concentration of H2O are preferred. 

A crystal growth process which is receiving more attention for certain compounds is mineralisation which, although involving a complex series of mechanisms, certainly includes a vapour-solid stage. It is not intended to discuss this route here but small crystals of many rare earth pnictides and chalcogenides have been grown in this manner. 

The structure of the interface on an atomic scale determines the kinetics of the crystal growth processes, and indeed of phase transformations in general. The details of the atomic motions which constitute the phase change are dependent on the environment of the atoms involved. In first-order phase transformations these atomic processes take place at an interface separating the two phases. Motion of the interface propagates the phase transformation. 

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