Crystallization, Precipitation, and Solidification

Although there are analogies between solidification and crystallization, there are also some important structural differences between the solid-melt interface (also called the solidification front) and the solid-solvent interface due to the differing concentration, viscosity, and temperature of the respective liquid phase. These may be summarized as follows  

In 1897, Friedrich Wilhelm Ostwald (1853-1932) published his now famous study of crystallization processes, which led to the Ostwald rule of stages or Ostwald step rule (Ostwald, 1897). Ostwald noticed that the course of transformation of unstable (or metastable) states into stable states normally occurs in stages, 

In subsequent experiments, using other crystal systems, such as ferrous sulfate and sodium hydrogen phosphate, it was similarly observed that the first crystallization product to form was the one most closely resembling the structure of the solvent (Nyvlt, 1995). For the case of citric acid, this is the monohydrate, which more closely resembles the aqueous structure. As the temperature of the solution is increased, the structure of the solvent, as well as the solubility of the crystal, changes, resulting in a more thermodynamically stable anhydrous product. This conversion between the kinetic and thermodynamic product occurs at a critical transition temperature, below which the structure of the solution favors the formation of the hydrated product. As the transition temperature is surpassed, the anhydrous product becomes favored. 

If we examine the competitive growth of multiple-size crystallites in solutions, we find the growth occurring along concentration gradients. The concentration gradients around the particles are explained with the Gibbs-Thomson equation  

Source Louis P. Hammett, Biographical Memoirs of the U.S. National Academy of Sciences, Vol. 45, 1974, pp. 203-226. 

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In this chapter we discuss preparative routes for inorganic materials in three basic types of systems involving the presence of a distinct solid-liquid interface those in which the liquid and solid phases are of the same chemical identity (solidification and vitrification processes), those in which the liquid and solid phases are not of the same chemical identity (crystallization, precipitation), and the special case in which the liquid phase is a pure ionic liquid or molten salt. Ionic liquids can serve as the solvent as well as a templating agent, and the liquid components may or may not become incorporated into the final solid product. We also discuss two areas where the distinct solid-liquid interface becomes somewhat blurred namely, sol-gel and solvothermal processes. 

We will be looking at kinetics in Chapter 6. But before we can do this we need to know what we mean by driving forces and how we calculate them. In this chapter we show that driving forces can be expressed in terms of simple thermodynamic quantities, and we illustrate this by calculating driving forces for some typical processes like solidification, changes in crystal structure, and precipitate coarsening. 

When the equilibrium mixture is permitted to cool rapidly, the heavier salt phase settles and solidifies, thus leaving the lighter metal phase on top as a liquid. Inasmuch as calcium metal starts to precipitate and settle before the solidification point of the salt phase is reached, some of the calcium is trapped in the salt phase and, together with the calcium which is dissolved by the molten salt, is not recovered in the following synthesis. Once the sodium chloride-calcium chloride layer has solidified, however, the precipitating calcium remains trapped in the metal layer. When the metal layer freezes, its composition consists of calcium crystals embedded in a sodium matrix which may be removed by preferential reaction with a lower alcohol.9... 

The oily 3-methyl-l-phenyl-1,3-butanedione(l g) is added to a solution of sodium (0.15 g) in methanol (3 ml), and the deep yellow solution is at once filtered and dropped into 5n-sulfuric acid (30 ml) cooled in ice-salt. The emulsion that is thus first formed must be whipped vigorously with a glass rod to cause solidification, so that most of the material becomes finely powdered and readily filterable if the oil globules once become large the product does not crystallize well and is unstable. The solid precipitate is at once filtered off, washed with very dilute hydrochloric acid, followed by ice-cold ethanol and then light petroleum, and is finally recrystallized from low-boiling light petroleum in a quartz vessel. The m.p. is 51° and the yield almost quantitative. 

One unique advantage of the current droplet manipulation method is that the actuation can be continued even after a solidification reaction occurred inside a droplet. As shown in Fig. 6, not only a liquid droplet but also droplets containing solid precipitates or hydrogel and even a solid sphere can be actuated on the BCD system. The biochemical solidification reaction inside a droplet is demonstrated as the formation of a calcium carbonate crystal which is related to an in vitro study of biomineralization of marine organisms. When the Na2C03 and CaCla droplets are coalesced, by the BCD actuation of the coalesced droplet, the mixing and solidification reaction is accelerated and completed in... 

In order to form a small region as shown in Fig. 1 (b), thickeners, such as attapulgite clay that uses a capillary effect [6] and carboxymethyl cellulose [7], a water-soluble polymer are added. However, inorganic hydrated salts separate water when melting occurs and they incorporate water when solidification occurs. This cycle is repeated thousands of times. Thus, the sodium sulfate crystals grow and precipitate. In order to prevent the... 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

Coaxial intergrowth is a paragenetic relation that describes crystals of two different species growing with a common axis the misfit ratios between the two crystals in the direction of the common axis are small, without exception. The formation of coaxial intergrowth can be understood to be one crystal conjunct to the other in an epitaxial relation, where both continue to grow. If a liquid of eutectic A-B component is solidified from one side (unidirectional solidification), crystals of the two phases A and B precipitate in dotted, columnar or lamellar (with common axis) form, and show unique textures for unidirectional solidification. This is a well known phenomenon in metallurgy. 

The microstructure of the multiphase media is often the product of phase transitions, e.g. (i) capillary condensation in the porous media, (ii) phase separation in polymer/polymer and polymer/solvent systems, (iii) nucleation and growth of bubbles in the porous media, (iv) solidification of the melt with a temporal three-phase microstructure (solid, melt, gas), and (v) dissolution, crystallization or precipitation. The subject of our interest is not only the topology of the resulting microstructured media, but also the dynamics of its evolution involving the formation and/or growth of new phases. 

Molten solutions can also be used for crystallization. Fluxes are selected that have a high solubility of the to-be crystallized material. Borate fluxes are used for some oxide systems, sodium sulfide fluxes are used for sulfide systems, and molten metal fluxes are used for carbide and nitride systems. In both the melt and flux systems, the solubility is highly temperature dependent. The solubility in flux systems is not particularly well known except for a few systems. The general solubility behavior is like that in liquid phase precipitation, which was discussed in Chapter 6. In melt solidification, the supersaturation, S, is given by... 

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