Aluminum , crystal growth rate

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Anhy2drous calcium sulfoaluminate is formed by calcination of lime, gypsum and bauxite. The active expansive ingredient, C A S is formed by solid-state reaction from mixtures of compounds composed of calcium oxide, aluminum oxide, sulfur trioxide gas formed during the calcination of gypsum, and bauxite. Crystal growth of CSAs is encouraged to proceed at a slow rate to preserve the potential force of expansion for extended periods [76],... 

In the presence of dissolved aluminum ions, at not too high a temperature, the diflusion rate of the iron ions in the magnetite lattice is low. Hence, nucleus formation proceeds rapidly relative to crystal growth. Therefore, small iron crystallites, about 30 nm, form with correspondingly large specific surfaces. The aluminum probably remains partly in the iron crystallite in the form of very small FeAl204 areas statistically distributed over the lattice [262], [263], [294], [296], where an FeAl204 molecule occupies seven a Fe lattice positions [101]. 

Figure 2.38 Effect of flow rate on the growth rate of single crystal of potassium aluminum sulfate to dodecahydrate.

Zeolite beta was synthesized with different silicon sources. The dissolution of the silicon sources and the reaction temperature obviously determined the crystallization rate of the zeolite. Silica sol reacted rapidly at 110, 140, and 170°C, while fumed silica and TEOS only reacted readily at 170°C. The aluminum content in the reaction gel determined the termination of the crystal growth, after all aluminum was consumed, the crystal growth stopped. The acidic properties of the obtained materials were very similar, but a significant influence of the silicon source on the particle size of the resulting zeolite was observed. 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

Slow solvent evaporation is a valuable method for producing crystals of high quality it is used primarily for small molecules. The rate of growth is manipulated by adjusting conditions (initial concentration, temperature, evaporation rate) so that crystals are obtained in a few days, not months or minutes. The rate of evaporation can be adjusted by covering the solution with a watch glass or a piece of film (aluminum or Parafilm ) with a number of small holes punched in it. The solution is then left to stand undisturbed. 

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