Nucleation Crystallization Specific

In contrast to chrysotile fibers, the atomic crystal stmcture of amphiboles does not inherentiy lead to fiber formation. The formation of asbestiform amphiboles must result from multiple nucleation and specific growth conditions. Also, whereas the difference between asbestiform and massive amphibole minerals is obvious on the macroscopic scale, the crystalline stmctures of the two varieties do not exhibit substantial differences. Nonfibrous amphiboles also exhibit preferential cleavage directions, yielding fiber-shaped fragments. 

Dehydration reactions are typically both endothermic and reversible. Reported kinetic characteristics for water release show various a—time relationships and rate control has been ascribed to either interface reactions or to diffusion processes. Where water elimination occurs at an interface, this may be characterized by (i) rapid, and perhaps complete, initial nucleation on some or all surfaces [212,213], followed by advance of the coherent interface thus generated, (ii) nucleation at specific surface sites [208], perhaps maintained during reaction [426], followed by growth or (iii) (exceptionally) water elimination at existing crystal surfaces without growth [62]. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Several factors must be considered when deciding whether a given precipitation reaction is a feasible basis for an analytical method solubility certainly is one, another relates to physical properties, and still another to chemical purity and stability. The physical properties of a precipitate are influenced by the mode of its formation as well as by the specific compound involved. Broadly speaking, precipitates and their properties depend on the processes of nucleation, crystal growth, and aging. This chapter deals with these topics and also the properties of colloidal precipitates. 

In this rather new field of zeolite synthesis [5], preliminary information on precursors in solution has been obtained by measuring the free fluoride ion concentration with a specific fluoride anion electrode. The mean number N of F bonded to one Si or other T atom (T = Al, Fe, Ga) has been computed and results are summarized in Table 2. No information on the exact nature of the species in solution is available discussed in the case of OH" type synthesis. Such a study has the potential of revealing some interesting information given the slower nucleation/crystal growth processes, the smaller number of metastable phases observed and the crystallization of almost defect-free zeolites in this medium [5]. F NMR could be added to the other spectroscopic characterization techniques for such solutions. 

Fast crystallization allows faster molding cycles or clearer extruded thin film. Nucleation speeds this crystallization process. Crystallization is a complex subject because there can be a downside as well as immediate economic advantage. Many different types of chemicals can nucleate crystallization. The eff ect is specific for the polymer and the chemical. Glass fibers, pigments, talc, etc. can nucleate crystallization for some polymers. 

Let us first consider the intramolecular nucleation. The specific free energy on the lateral surfaces of PE crystals was estimated to be ll.Serg/cm, and its specific free... 

Confined crystallization is a phenomenon that occurs in droplet dispersions, polymer blends, block copolymers, and thin films. Confinement has many consequences on the nucleation and crystallization behavior. Among the most notorious are the production of fractionated crystallization and the possibility of isolating crystallizable phases whose nucleation may be very different heterogeneous, superficial, or homogeneous nucleation. In specific cases confinement can also lead to crystal modifications for polymorphic polymers. 

In spite of these obstacles, crystallization does occur and the rate at which it develops can be measured. The following derivation will illustrate how the rates of nucleation and growth combine to give the net rate of crystallization. The theory we shall develop assumes a specific picture of the crystallization process. The assumptions of the model and some comments on their applicability follow ... 

Physical properties of the acid and its anhydride are summarized in Table 1. Other references for more data on specific physical properties of succinic acid are as follows solubiUty in water at 278.15—338.15 K (12) water-enhanced solubiUty in organic solvents (13) dissociation constants in water—acetone (10 vol %) at 30—60°C (14), water—methanol mixtures (10—50 vol %) at 25°C (15,16), water—dioxane mixtures (10—50 vol %) at 25°C (15), and water—dioxane—methanol mixtures at 25°C (17) nucleation and crystal growth (18—20) calculation of the enthalpy of formation using semiempitical methods (21) enthalpy of solution (22,23) and enthalpy of dilution (23). For succinic anhydride, the enthalpies of combustion and sublimation have been reported (24). 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

Purely physical laws mainly control the behaviour of very large particles. Further down the particle size range, however, specific surface area, i.e. surface area per unit mass, increases rapidly. Chemical effects then become important, as in the nucleation and growth of crystals. Thus, a study of particulate systems within this size range of interest has become very much within the ambit of chemical engineering, physical chemistry and materials science. 

---